This invention relates to silicon capacitive pressure sensors, and more particularly to a silicon capacitive pressure sensor having a silicon substrate with a mesa formed therein, the mesa having an upper surface with a generally concave shape.
In the art of silicon capacitive pressure sensors, it is known to provide such a sensor as a single sensing element. Prior art single element silicon capacitive pressure sensors typically comprise a pair of parallel conductive silicon plates. A borosilicate glass spacer is deposited onto one of the plates, and the second plate is attached to the glass spacer by a field-assisted, vacuum bonding process. This forms an evacuated chamber within the opposing conductive plates and spacer. The opposing silicon plates comprise the plates of a pressure variable capacitor. See, for example, U.S. Pat. Nos. 4,415,948, 4,405,970 and 4,530,029. Examples of electronic circuitry used to process the sensor output signals indicative of sensed pressure are described and claimed in U.S. Pat. Nos. 4,743,836 and 4,517,622.
In a silicon capacitive pressure sensor, one conductive silicon plate forms a diaphragm that flexes inwardly in the presence of fluid pressure applied to the outside surface of the diaphragm that is greater in magnitude than the pressure (usually vacuum) in the chamber. The second conductive silicon plate forms a substrate that is normally held rigid. The deflection of the diaphragm causes a variation in the distance between the plates, thereby varying the capacitance of the plates. Thus, the capacitive pressure sensor is operative to transduce pressure variations into corresponding capacitive variations. The borosilicate glass spacer serves not only to separate the plates, but also to seal the vacuum chamber therebetween. The silicon diaphragm and substrate are normally doped to make them appropriately electrically conductive.
These pressure sensing devices are particularly well suited for miniaturization due to the fine dimensional control achievable using the semiconductor and thin-film technologies. Microcircuit technology can produce a large number of pressure sensors fabricated from a single silicon wafer. They are also well suited to the measurement of small differential pressures in various commercial and aerospace applications.
However, in any silicon capacitive pressure sensor, parasitic capacitance is a limitation on the accuracy of the sensor. This is because such parasitic capacitance may result in an overall long-term drift (20 years) of the sensor output. This is especially true in high accuracy (0.05% or 500 ppm) pressure sensing applications at high temperatures (120.degree. C.). This limiting factor may make some sensor designs unsuitable for demanding aerospace applications, such as electronic engine controls ("EECs") and air data computers ("ADCs").
Parasitic capacitance is the inherent capacitance of the non-pressure sensitive interstices of the sensor structure. For example, the parasitic capacitance of the borosilicate glass spacer may comprise upwards of 50% of the total capacitance of the sensor. Such parasitic capacitance reduces the sensor gain because it adds in parallel to the pressure sensitive capacitance of the sensor. This reduces both the dynamic range of the sensor and its sensitivity to pressure changes. Thus, a large effort has been placed in the past on reducing such capacitance through variations in the design of the sensor architecture.
However, parasitic capacitance is inherent in any physical structure and there is a minimum practically achievable value that may still be unacceptable in high sensitivity sensing applications. U.S. Pat. No. 4,405,970 discloses a method of reducing the parasitic capacitance in a silicon capacitive pressure sensor by providing specific borosilicate glass structures that separate fixed portions of the two capacitive plates at a relatively long distance from each other. Another approach to reducing the parasitic capacitance is disclosed in U.S. Pat. No. 4,467,394, in which a three-plate device is utilized that, when combined with appropriate signal processing circuitry, eliminates the parasitic capacitance from the measurement. A further approach to eliminating the parasitic capacitance is disclosed in U.S. Pat. No. 4,951,174.
It is known in the prior art of silicon capacitive pressure sensors to employ a substrate and diaphragm that are both uniformly planar in design and to arrange them in a parallel relationship. Also, it is known to form a planar mesa surface on a parallel planar surface of the substrate. Thus, the parallel relationship of the diaphragm and substrate provides for a uniform spacing therebetween, which further translates into a uniform capacitance therebetween, with no flexing of the diaphragm.
When the fluid pressure applied to an outer surface of the diaphragm exceeds the fluid pressure in the gap between the diaphragm and substrate, the diaphragm flexes toward the substrate. However, the outer edge portions of the diaphragm are fixedly attached to the dielectric glass spacer. Thus, the center of the diaphragm flexes the greatest amount toward the substrate, while the amount of flexing progressively decreases outwardly away from the center of the diaphragm toward its outer edges. At the same time, the mesa surface of the substrate remains planar. Thus, the distance between the planar mesa and the flexed diaphragm varies throughout the gap. This gap variation translates into a variation in the capacitance at different spatial locations between the diaphragm and substrate. More importantly, the non-uniform gap spacing translates into reduced sensitivity of the sensor to changes in fluid pressure applied to the diaphragm.
Accordingly, it is a primary object of the present invention to provide a silicon capacitive pressure sensor having improved sensitivity to the pressure of a fluid applied thereto.
It is a general object of the present invention to provide a silicon capacitive pressure sensor having an increased capacitance over prior art designs for a given value of a pressure of a fluid applied thereto.
It is a further object of the present invention to provide a silicon capacitive pressure sensor having a substrate with a generally curved mesa surface that approximates the curvature of the diaphragm when the diaphragm is flexed by a fluid pressure applied thereto.
It is a still further object of the present invention to provide a silicon capacitive pressure sensor that has a relatively uniform air gap between the diaphragm when flexed and a curved mesa surface of the substrate.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.